PAR3 is a cofactor for PAR4 activation by thrombin

Abstract

Identification of the mechanisms by which the coagulation protease thrombin activates platelets is critical for understanding haemostasis and thrombosis. Thrombin activates cells at least in part by cleaving protease-activated G-protein-coupled receptors (PARs). PAR3 and PAR4 are thrombin receptors expressed in mouse platelets. Inhibition of thrombin binding to mPAR3 (ref. 4) and knockout of the mPAR3 gene inhibited mouse platelet activation at low but not high concentrations of thrombin. Thus PAR3 is important for thrombin signalling in mouse platelets. Expression of human PAR3 in heterologous expression systems reliably resulted in responsiveness to thrombin. Curiously, despite its importance for the activation of mouse platelets by thrombin, mouse PAR3 (mPAR3) did not lead to thrombin signalling even when overexpressed. We now report that mPAR3 and mPAR4 interact in a novel way: mPAR3 does not itself mediate transmembrane signalling but instead functions as a cofactor for the cleavage and activation of mPAR4 by thrombin. This establishes a paradigm for cofactor-assisted PAR activation and for a G-protein-coupled receptor's acting as an accessory molecule to present ligand to another receptor.

Thrombin induced connective tissue growth factor expression in rat vascular smooth muscle cells via the PAR-1/JNK/AP-1 pathway

“…Although subtypes of thrombin-responsive PARs, PAR-1, PAR-3, and PAR-4, are present and functionally active in VSMCs, PAR-1 has the highest affinity for thrombin [31] . PAR-1 is the prototypic thrombin receptor and the main isoform involved in VSMC neointimal formation and restenosis in vivo [32] , whereas PAR-3 appears to function as a cofactor for PAR-4 [33] . In this study, we found that a PAR-1 antagonist (SCH79797) significantly inhibited thrombin-induced CTGF expression, while a PAR-4 antagonist (tcY-NH 2 ) had no effect.…”

Protease-Activated Receptors as Therapeutic Targets in Visceral Pain

“…Very little is known about PAR 3 since in cells transfected with this receptor, no intracellular signal transduction has been detected in response to either thrombin or its tethered ligand peptide [9]. Interestingly, one study showed the implication of PAR 3 as an accessory protein binding thrombin by its hirudin-like domain for PAR 4 activation [10]. Like PAR 4 was the most recent member of the PARs family to be cloned, very little is known about the physiological and pathophysiological importance of this receptor in visceral pain.…”

Glycosylation and the activation of proteinase‐activated receptor 2 (PAR2) by human mast cell tryptase

“…There are currently three active members of the PAR family, PAR 1, 2 and 4 (Bohm et al, 1996;Ishihara et al, 1997;Vu et al, 1991;Kahn et al, 1998;Xu et al, 1998). Family member, PAR 3 , has been found to be a co-factor for PAR 4 and as such may not be considered as an active receptor in its own right (Nakanishi-Matsui et al, 2000). Interestingly, peptides corresponding to the ®rst ®ve to six amino acids of the tethered ligand will activate these receptors independently of proteolysis.…”